Satellite & Terrestrial Link Budget
Compute probabilistic link margins using ITU-R propagation models (P.618 rain, P.676 gaseous, P.840 cloud). Enter EIRP, G/T, frequency, and site location. Get a full line-by-line budget, availability curve, and Monte Carlo confidence intervals over rain, pointing, and EIRP/G·T uncertainties.
How It Works
A link budget calculates whether received signal power is sufficient for reliable communications. The fundamental equation is:
C/N₀ = EIRP + G/T − FSPL − A_rain − A_gas − A_cloud − 10 log₁₀(k)
where FSPL = 20 log₁₀(4πdf/c) and k = 1.38 × 10⁻²³ J/K (Boltzmann constant).
Rain attenuation (ITU-R P.618) dominates at Ku-band (12 GHz) and above. The model computes specific attenuation γ_R = k × R^α from the rain rate at 0.01% availability for the site's climate zone, then integrates over the slant path geometry through the rain layer.
Gaseous absorption (ITU-R P.676) from oxygen and water vapour is significant above 20 GHz and at low elevation angles.
The Monte Carlo sweeps EIRP (±0.3 dB), G/T (±0.3 dB), pointing loss (exponential mean 0.2 dB), rain rate (log-normal σ = 0.5), and scintillation (normal σ = 0.4 dB) to produce margin confidence intervals at the target availability.
Related Calculators
FAQ
What is link availability and why does 99.5% matter?+
99.5% availability means the link is operational for 99.5% of the year — it experiences outages for 0.5% of time, which is 43.8 hours/year. This is typically quoted as an annual statistic and is dominated by rain fade events. The remaining 0.05% (4.4 hours/year) represents exceptional storm events. Broadcast satellite services typically target 99.5–99.7%. Critical infrastructure links target 99.99% or better.
Why does lowering the elevation angle increase rain attenuation?+
The satellite signal passes through more of the atmosphere at shallow elevation angles. The slant path length through the rain layer scales as 1/sin(elevation). At 5° elevation, the path is 11× longer than at 90°. This dramatically increases the rain attenuation. For low-elevation or terrestrial links, rain attenuation is the binding constraint for availability.
What do the Monte Carlo confidence bands mean?+
The p5/p50/p95 bands represent the distribution of link margin outcomes when all uncertain parameters are drawn from their statistical distributions. The p5 margin is exceeded by 95% of operating scenarios — it is the conservative design point for worst-case margin allocation. If p5 margin is negative, 5% of operating scenarios will be outages in addition to the rain fade outage budget.